One thread stuck at 100% usage due to connection flood? about qbittorrent HOT 2 OPEN

what exactly is happening that uses so much CPU?

Probably a DDoS or another kind of attack.

does qBt/libtorrent have any built-in protections against this?

qB restricts a number of active connections (Settings > Connection > Global maximum number of connections) to 500 by default. This could not really help if the connections are fast pacing though and total count doesn't make it enough to hit the limit.

could this be a bad actor? if so, what does he gain from this?

Yes. Who knows.

has anyone else seen this?

Yep, different kind of attacks will happen, especially if you have a static IP. Torrent clients basically advertise their existance to the whole world via DHT.

So you have no other choice than utilizing firewalls. The client itself also supports IP filters, but I guess it will continue to drain CPU to reject connections (and also spams into the log), so OS-level firewall it the way to go I believe.
Changing your client port could help, at least temporarily, but chances are attackers will find the new port. Switching IP also could help, if possible.

from qbittorrent.

qB restricts a number of active connections (Settings > Connection > Global maximum number of connections) to 500 by default.

Indeed and I have it set to 500. It doesn't seem to have helped in this case.

from qbittorrent.